ci: template custom runner names by repo (#27024)

## Why

These workflows currently hard-code the `codex` runner group and custom
runner labels. That makes the same workflow definitions less portable
across repository copies or renamed repos, even though the runner fleet
follows the repository name scheme. Template the runner identities from
the repository name so `openai/codex` still resolves to the existing
`codex-*` runners while other repos can use their own `<repo>-*` runner
names.

## What Changed

- Replaced custom runner `group` values such as `codex-runners` with
`${{ github.event.repository.name }}-runners`.
- Replaced custom runner labels such as `codex-linux-x64` and
`codex-windows-arm64` with `${{ github.event.repository.name }}-...`.
- Covered direct `runs-on` objects, matrix `runs_on` entries, reusable
workflow runner inputs, and release runner labels.

## Verification

- Parsed all `.github/workflows/*.yml` files as YAML with Ruby.
- Searched `.github/workflows` to confirm no hardcoded runner-field
`codex-runners` or `codex-*` labels remain.

.github/workflows/bazel.yml

@@ -152,8 +152,8 @@ jobs:
           - 3
           - 4
     runs-on:
-      group: codex-runners
-      labels: codex-windows-x64
+      group: ${{ github.event.repository.name }}-runners
+      labels: ${{ github.event.repository.name }}-windows-x64
     name: Bazel test on windows-latest for x86_64-pc-windows-gnullvm shard ${{ matrix.shard }}/4
     environment:
       name: bazel
@@ -267,8 +267,8 @@ jobs:
     if: github.event_name == 'push' && github.ref == 'refs/heads/main'
     timeout-minutes: 40
     runs-on:
-      group: codex-runners
-      labels: codex-windows-x64
+      group: ${{ github.event.repository.name }}-runners
+      labels: ${{ github.event.repository.name }}-windows-x64
     name: Bazel test on windows-latest for x86_64-pc-windows-gnullvm (native main)
     environment:
       name: bazel
@@ -357,8 +357,8 @@ jobs:
           - os: windows-latest
             target: x86_64-pc-windows-gnullvm
             runs_on:
-              group: codex-runners
-              labels: codex-windows-x64
+              group: ${{ github.event.repository.name }}-runners
+              labels: ${{ github.event.repository.name }}-windows-x64
     runs-on: ${{ matrix.runs_on || matrix.os }}
     name: Bazel clippy on ${{ matrix.os }} for ${{ matrix.target }}
     environment:
@@ -453,8 +453,8 @@ jobs:
           - os: windows-latest
             target: x86_64-pc-windows-gnullvm
             runs_on:
-              group: codex-runners
-              labels: codex-windows-x64
+              group: ${{ github.event.repository.name }}-runners
+              labels: ${{ github.event.repository.name }}-windows-x64
     runs-on: ${{ matrix.runs_on || matrix.os }}
     name: Verify release build on ${{ matrix.os }} for ${{ matrix.target }}
     environment: